Solid polymer electrolyte fuel cells using a polymer electrolyte membrane are anticipated to be practically used soon as the power source for household use and electric cars, or for mobile devices such as cell phones and laptop computers.
A solid polymer electrolyte fuel cell (hereinafter simply referred to as “fuel cell”) has at least one unit fuel cell including: a membrane electrode assembly (hereinafter referred to as “MEA”); and a pair of separators each of which is disposed on each side of the MEA. The MEA includes an anode, a cathode, and a polymer electrolyte membrane interposed therebetween. The anode and the cathode each include a catalyst layer and a gas diffusion layer.
The anode is bonded to one face in the thickness direction of the polymer electrolyte membrane, and the cathode is bonded onto the other face in the thickness direction thereof. The pair of separators are disposed so as to sandwich the MEA from both faces thereof in the thickness direction. In the fuel cell, power is generated by supplying fuel such as hydrogen gas to the anode and supplying an oxidant such as air to the cathode.
In the case of using a fuel cell as the power source for use other than in small-size devices such as cellular phones, a fuel cell system is configured, in which a fuel cell is provided together with respective supplying devices for air and fuel, and a controlling device for power generation. In the fuel cell system, a fuel cell stack including a plurality of unit fuel cells is used. The respective supplying devices supply air to the cathode and fuel to the anode. As supplying devices, specifically, blowers, pumps, and the like are used. The controlling device, for example, controls: power generated in the fuel cell stack; temperature of the fuel cell stack; and the supplying and stopping of air and of fuel.
However, there are a number of problems for enabling practical use of a fuel cell.
One problem concerns the control of humidification in the unit fuel cell. If the polymer electrolyte membrane used in the unit fuel cell is not in a humidified state, proton conductivity would degrade drastically. Due to the above, in the case where the polymer electrolyte membrane is in a state of lacking humidification, the output power of the unit fuel cell, and further, the output power of the fuel cell stack would degrade drastically. In addition, if the unit fuel cell or the fuel cell stack is operated when the polymer electrolyte membrane is in a state of lacking humidification, there would be an overvoltage at the electrode, thus causing problems such as a rise in electrode potential and side reactions. It is becoming apparent that such problems promote degradation of the catalyst material and the carbon material in the electrode, the polymer electrolyte membrane, and the like.
Normally, the state of humidification in the unit fuel cell is not sufficient immediately after production of a fuel cell stack including a plurality of unit fuel cells. Therefore, the unit fuel cell needs to be humidified in the post-process. In addition, in cases such as where the fuel cell system is in a hibernating state for a long period of time, moisture contained in the unit fuel cell may gradually dissipate to the outside, thus causing the unit fuel cell to be in a state of lacking humidification. In such a case, normally, a method is used in which power generation is conducted to humidify the unit fuel cell.
In the case where fuel is hydrogen gas, normally, hydrogen gas supplied to the anode and air supplied to the cathode each passes through a humidifier to be humidified. In addition, in the case where fuel is an aqueous methanol solution, moisture of the aqueous methanol solution is directly supplied to the anode. In either case, if fuel is supplied for conducting power generation, moisture would be supplied into the unit fuel cell. In addition, water is produced in the cathode by an electrode reaction caused when power is generated. Due to the above reason, the unit fuel cell is humidified by conducting power generation.
In a method for humidifying a unit fuel cell by conducting power generation, humidification is possible in a relatively short period of time. However, even if the time required for humidification is short, from the aspect of life characteristics of the unit fuel cell, it is not favorable to conduct power generation when the unit fuel cell is in a state of lacking humidification.
As a solution for such a problem, a method can be considered in which a unit fuel cell is humidified without conducting power generation. For example, Japanese Laid-Open Patent Publication 2000-003718 (Document 1) proposes: introducing into the unit fuel cell, water or a weakly acidic aqueous solution with a higher temperature than the operating temperature of the fuel cell stack; cleansing with water or a weakly acidic aqueous solution after alcohol is introduced; and the like. The main object of the technique disclosed in Document 1 is considered to be humidification of a fuel cell stack during the process immediately after production of the fuel cell stack. The technique disclosed in Document 1 also enables humidification of a unit fuel cell after a long period of hibernation.
Japanese Laid-Open Patent Publication 2005-294173 (Document 2) proposes humidifying a unit fuel cell at a temperature of 80° C. to 200° C. with a relative humidity of 50% to 100%. Document 2 does not specifically describe how humidification is conducted, but discloses about warming and humidifying fuel gas within the above ranges.